1. Field of the Invention
This invention relates generally to digital receivers for use with pseudo random noise (PRN) encoded signals such as those used in satellite ranging systems.
2. Background Information
There are a number of satellite ranging systems that are currently deployed and additional systems are to be deployed in the near future. Each of these configurations is based upon transmission of ranging signals in particular frequency bands. More specifically, the present United States Global Positioning System (GPS) is based on transmission of ranging signals in two frequency bands known as L1, which is at a center frequency of 1575.42 MHz and L2, centered at 1227.6 MHz. To enhance the reliability and availability of this system, additional GPS signal structures are planned (e.g. L5, L2C). In addition, other satellite ranging systems are being deployed or have been deployed such as that of the Russian Federation, i.e., GLONASS (with two signal structures: G1 and G2), and the European GALLILEO system with multiple signal structures (referred to herein as: E1, E2 . . . E5, etc.)
The system satellites transmit precisely timed signals that contain a number of components, namely, a plurality of pseudo-random noise (PRN) codes and data. The signals allow for precise determination of latitude, longitude, elevation and time. A digital receiver operating in a known manner, receives the PRN-encoded signals and essentially synchronizes local versions of the transmitted codes to the received codes, that is, the receiver tracks the received codes, to determine time differences of arrival and Doppler measurements. The digital data, which consists of information such as the satellite ephemeris, (i.e. position, current time of day, and system status information), is also transmitted by each satellite on at least one carrier frequency as a low frequency (typically 50 Hertz) signal. After synchronization of the local PRN codes, and their carriers, the receiver obtains the data. The receiver then uses the information provided by the data and the times of arrival and so forth to produce pseudoranges for the respective satellites in view and to determine its global position using the pseudoranges.
As noted, a satellite ranging signal receiver receives a composite signal consisting of one or more of the signals transmitted by the satellite within view (within a direct line of sight) as well as noise and interfering signals. By determining the transmission time from at least four satellites and knowing each satellite's ephemeris, the receiver can calculate the pseudoranges and thus its three-dimensional position, its velocity and the precise time of day.
When calculating heading information of a mobile device, such as a boat, aircraft or other vehicle, several receivers and corresponding antennas are located on the vehicle, spaced apart from one another. The antennas receive signals from the same set of satellites and determine their global positions as described above. Once the position of each antenna is known, the position information can be used to calculate a precise directional heading of the boat or other vehicle upon which the antennas are mounted. Alternately and preferably interferometric differences in the measured phase data can be used to determine attitude and relative orientation of antennas, as is well know in the art. However, a separate receiver has typically been required for each antenna being used to make such a heading calculation, thus leading to a costly system for receiving and analyzing inputs from each of the antennas used to provide the requisite information.
In U.S. Pat. No. 6,844,847 entitled BOAT POSITIONING AND ANCHORING IN A SYSTEM, of Gounon, which issued on Jan. 18, 2005, a receiver is described that includes a multiplexer which multiplexes the signals from two separate antennas together such that the receiver can separately utilize the signals from each antenna to track the respective PRN codes and determine the global positions of the antennas. However, this receiver tracks the codes in the signals provided by a given antenna only half of the time and thus, the tracking operations are susceptible to loss of phase lock. If phase lock is lost, the receiver must re-align the local codes in order to determine the antenna positions reliably, and the results of the heading calculations are therefore delayed or may be interrupted entirely.
There remains a need, therefore, for a receiver which, inter alia, receives and simultaneously processes signals from more than one antenna and uses information from each respective antenna to calculate the heading of an associated vehicle. There remains a further need for a receiver architecture which is of a reduced size and cost, and which accommodates multiple antenna signals at a nominal increase in receiver complexity.